Flexible printed wiring board

ABSTRACT

An object of the present invention is to provide a flexible printed wiring board which relaxes stress concentration in the flexible printed wiring board during production steps, thereby preventing wire breakage in inner lead portions and cracking in solder resist which would otherwise be caused during mounting of devices such as IC chips and LSI chips. The flexible printed wiring board of the present invention includes an insulating layer; a wiring pattern formed of a plurality of wirings being juxtaposed, which wiring pattern is formed through patterning a conductor layer stacked on at least one surface of the insulating layer and on which wiring pattern a semiconductor chip is to be mounted; and grid-like dummy patterns formed in a blank area where the wiring pattern is not provided, wherein the dummy patterns are formed in a width direction generally symmetrically with respect to the longitudinal direction of the flexible printed wiring board.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/300,399 filed Dec. 15, 2005, the above-noted application incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexible printed wiring board such asa TAB tape or a flexible printed circuit (FPC), which wiring board haswiring patterns and serves as a substrate for supporting semiconductorchips. More particularly, the invention relates to a wide flexibleprinted wiring board which has a large surface area per wiring patternand which is used in a plasma display panel (PDP) or a similar device.

2. Description of the Related Art

Development of the electronics industry has been accompanied by sharpdemand for printed wiring boards for mounting electronic devicesthereon, such as IC (integrated circuit) chips and LSI (large-scaleintegrated circuit) chips. In the field of plasma display panels (PDPs),wider and more large printed wiring boards are used more than that ofliquid crystal devices (LCDs) because the printed wiring boards for PDPshave wider wirings in wider pitch than those for LCDs in order to use ea higher voltage.

Such tape-form flexible printed wiring boards having a large width havedrawbacks. For example, the flexible printed wiring boards are deformedduring conveyance thereof, particularly uncoiling and coiling upthereof, thereby causing stress concentration in specific portions ofthe wiring boards. Therefore, after completion of a mounting step suchas IC chip bonding, wire breakage in inner lead portions and cracking insolder resist occur.

More specifically, as shown in FIG. 4, after completion of productionsteps or product inspection, a flexible printed wiring board 1 is guidedby means of guide rollers 2 and 3 and then coiled up by a reel 4. Duringthe above conveyance, the flexible printed wiring board 1 is uncoiled tobe conveyed in a horizontal direction and, via the guide roller 2,downward to the bottom of a U-shaped section 5, which acts as a buffersection. After passage through the bottom of the U-shaped section 5, theflexible printed wiring board 1 is conveyed upward to the guide roller3. Since the direction of bending at the guide rollers 2 and 3 differsfrom that at the U-shaped section 5, stress tends to be concentrated inspecific portions. In addition, as shown in FIG. 5, the flexible printedwiring board 1 is generally warped widthwise with thewiring-pattern-provided surface 1 a inside. Also, the wider the width ofthe tape, the higher the degree of warpage. When such warpage isstraightened at the guide rollers 2 and 3, stress is also concentratedin specific portions of the flexible printed wiring board 1.

In an attempt to solve the drawbacks of such flexible printed wiringboards, Japanese Patent No. 3350352 (Claims, paragraphs such as [0005])proposes a carrier tape exhibiting enhanced resistance to stress. In theflexible printed wiring board, wirings provided on the film substratehave almost the same total length of non-parallel sections so as toattain uniform stress resistance in both the center and the periphery ofthe film.

However, the aforementioned drawbacks in relation to stressconcentration are considered to be attributed to marginal portions whereno wiring pattern is provided.

In an attempt to prevent generation of cracks and wire breakage inwiring patterns caused by repeated temperature changes effected duringdevice mounting, Japanese Patent Application Laid-Open (kokai) No.11-45913 (Claims, paragraphs such as [0010] and [0030]) proposes aflexible printed wiring board having a dummy pattern around a devicehole, the dummy pattern being electrically unconnected with the wiringpattern. The dummy pattern assumes a shape similar to that of the wiringpattern and is provided such that substantial uniformity in patterndensity and thermal expansion coefficient over the film carrier isattained.

However, even when such a dummy pattern is provided, the aforementionedproblems in the production of flexible printed wiring boards are notcompletely solved.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a flexible printed wiring board which relaxes stressconcentration in the flexible printed wiring board during productionsteps, thereby preventing wire breakage in inner lead portions andcracking in solder resist which would otherwise be caused duringmounting of devices such as IC chips and LSI chips.

Accordingly, in a first mode of the present invention, there is provideda flexible printed wiring board comprising

an insulating layer;

a wiring pattern formed of a plurality of juxtaposed wirings, whichwiring pattern is formed through patterning a conductor layer laminatedon at least one surface of the insulating layer and on which wiringpattern a semiconductor chip is to be mounted; and

grid-like dummy patterns formed in a blank area where the wiring patternis not provided,

wherein the dummy patterns are formed in a width direction generallysymmetrically with respect to the longitudinal direction of the flexibleprinted wiring board.

In the flexible printed wiring board of the first mode, the grid-likedummy patterns are formed in a width direction generally symmetricallywith respect to the longitudinal direction of the flexible printedwiring board. Therefore, the flexible printed wiring board has uniformflexibility, and stress generated, for example, upon straightening orreversing of warpage during conveyance of the wiring board in productionthereof is mitigated and released. Thus, there can be prevented wirebreakage in inner lead portions and cracking in solder resist, whichwould otherwise be caused during mounting of devices such as IC chipsand LSI chips.

In a second mode of the present invention, each of the grid-like dummypatterns may be provided in a direction of juxtaposition of thejuxtaposed wirings.

The flexible printed wiring board of the second mode has grid-like dummypatterns each having wirings which extend in the same direction as theextending direction of wirings forming the wiring pattern. Therefore,more uniform flexibility of the wiring board can be effectivelyattained, and stress generated, for example, upon straightening orreversing of warpage during conveyance of the wiring board duringproduction thereof is mitigated and released. Thus, there can beprevented wire breakage in inner lead portions and cracking in solderresist, which would otherwise be caused during mounting of devices suchas IC chips and LSI chips.

In a third mode of the present invention, the wiring pattern may beformed such that the wirings are provided along the longitudinaldirection of the flexible printed wiring board, and the dummy patternsare provided on both sides of the wiring pattern with respect to thedirection of juxtaposition of the juxtaposed wirings.

In the flexible printed wiring board of the third mode, rigidity of theblank areas on both sides of the wiring pattern with respect to thedirection of juxtaposition of the juxtaposed wirings is controlled,thereby attaining uniform flexibility. Thus, concentrated stress can beeffectively relaxed during conveyance of the wiring board in productionthereof.

In a fourth mode of the present invention, the flexible printed wiringboard may be a wide wiring board having a width of 48 mm or more.

Even though the flexible printed wiring board of the fourth mode has awidth of 48 mm or more, concentrated stress can be relaxed duringconveyance of the wiring board in production thereof.

In a fifth mode of the present invention, the flexible printed wiringboard may be configured such that

a device hole is provided in an area where the semiconductor chip is tobe mounted, the area corresponding to longitudinal mid portions of thewirings of the wiring pattern, and

that a flexural slit is provided for imparting appropriate bending tothe board, the flexural slit including a plurality of component slitsthat are aligned so as to form a row running in a direction ofjuxtaposition of the juxtaposed wirings, and the flexural slit beingprovided on at least one of two opposing areas divided by the devicehole along the longitudinal direction of the flexible printed wiringboard,

wherein adjacent component slits are separated from each other by astrut portion therebetween, and the strut portion is disposed away, inthe width direction of the flexible printed wiring board, from theposition of each end of the device hole with respect to a direction ofjuxtaposition of the juxtaposed wirings.

In the flexible printed wiring board of the fifth mode, concentration ofstress at each end along a long side of the device hole can be preventedduring conveyance of the flexible printed wiring board. Stressconcentration in the inner leads provided along the device hole uponbending can be relaxed through the flexural slit(s).

In a sixth mode of the present invention, each of the dummy patterns maybe separated by the mediation of an area where the flexural slit hasbeen provided.

In the flexible printed wiring board of the sixth mode, a dummy patternis provided in each of the areas separated by the mediation of theflexural slit(s), whereby stress concentration can be effectivelyprevented.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood with reference to the following detailed descriptionof the preferred embodiments when considered in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic plan view of a film carrier tape (3-layer TABtape) according to Embodiment 1 of the present invention;

FIG. 2 is a schematic plan view of a film carrier tape (3-layer TABtape) according to Embodiment 2 of the present invention;

FIG. 3 is a schematic plan view of a film carrier tape (3-layer TABtape) according to Embodiment 3 of the present invention;

FIG. 4 is a sketch showing production of a flexible printed wiringboard; and

FIG. 5 is a sketch showing warpage of a flexible printed wiring board.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the flexible printed wiring board of present inventionwill next be described. Needless to say, the present invention is notlimited to the below-described production methods and modes of use.

Embodiment 1

FIG. 1 schematically shows a plan view of a flexible printed wiringboard of Embodiment 1. Although FIG. 1 shows a portion of the flexibleprinted wiring board corresponding to one film carrier product, aflexible printed wiring board is generally produced in a continuousmanner as a long tape product. In general production steps, electronicdevices such as IC chips are mounted on a tape substrate while the tapesubstrate is conveyed. In some cases, the device-mounted tape substrateis cut to provide product pieces, and in other cases, the tape substrateis cut, followed by mounting device chips. Hereinafter, the embodimentwill be described, taking a tape-form flexible printed wiring board asan example.

The flexible printed wiring board 10 of Embodiment 1 shown in FIG. 1 isa TAB tape formed of a tape-form insulating film 11 and a plurality ofwiring patterns 12 which are continuously formed on one surface of theinsulating film. As shown in FIG. 1, the insulating film 11 has, alonglongitudinal edges thereof, rows of sprocket holes 13 for conveying thefilm at predetermined intervals, and also has a device hole 14 in anarea where an electronic device is to be mounted. In addition, the boardhas flexural slits 15 and 16 which are provided at specific positions onboth sides of the device hole 14 with respect to the longitudinaldirection of the tape. The flexural slits 15 and 16 are provided forregulating flexibility, and each flexural slit is formed of a pluralityof component slits 17 (3 component slits in Embodiment 1) extending inthe width direction of the tape and connected with one another by themediation of narrow strut portions 18. The flexural slits 15 and 16 maybe provided on one side of the device hole 14, or may be omitted.

The wiring pattern 12 has a plurality of wirings which extend in thelongitudinal direction of the tape. In Embodiment 1, the segments of thewirings extending from the upper section of FIG. 1 to the device hole 14are referred to as output wirings 21, whereas the segments of thewirings extending from the device hole 14 to the lower section of FIG. 1are referred to as input wirings 22. Terminal portions of wirings 21 and22 which abut the device hole 14 are referred to as inner leads 21 a and22 a, respectively, whereas those extending to the tape segment edgesare referred to as outer leads 21 b and 22 b, respectively. Generally,the areas other than the inner and outer leads are covered with a solderresist layer (not illustrated).

In the flexible printed wiring board 10 of the present invention, dummypatterns 23 to 26 whose dummy wirings extend in a grid-like manner inthe longitudinal direction of the tape are provided in blank areas A toD, which are present on both sides of the wiring pattern 12 in the widthdirection of the tape. The dummy patterns 23 to 26 are formed from theconductor layer, which provides the wiring pattern 12. The dummy wiringsof the dummy patterns 23 to 26 are not electrically connected with thewiring pattern 12. The dummy patterns 23 to 26 may or may not be coveredwith a solder resist layer.

No particular limitation is imposed on the areas where the dummypatterns 23 to 26 are to be provided and on the width and pitch of thedummy wirings, and these conditions may be appropriately predeterminedin consideration of the shape of the wiring pattern 12 and wiringdensity. Specifically, the width and pitch of the dummy wirings are notparticularly limited, so long as the wiring pattern 12 and the dummypatterns 23 to 26, as a whole, provide rigidity, particularly in thewidth direction, of the highest possible degree of uniformity.

The insulating film 11 employed in the present invention may be formedfrom a material having flexibility as well as resistance to chemicalsand heat. Examples of such a material for providing the insulating film11 include polyester, polyamide, polyimide, BT resin, and liquid crystalpolymers. Among them, an aromatic polyimide (all repeating units beingaromatic) having a biphenyl skeleton (e.g., Upilex, product of UbeIndustries, Ltd.) is preferred. The insulating film 11 generally has athickness of 25 to 125 μm, preferably 50 to 75 μm.

The wiring patterns 12 are provided on one side of the insulating film11 having the device hole 14 and the sprocket holes 13 and generallyhave been fabricated by patterning a conductor layer formed of conductorfoil of copper or aluminum. Such a conductor layer may be directlylaminated on the insulating film 11, or may be formed through pressing(e.g., thermal pressing) by the mediation of an adhesive layer. Theconductor layer has a thickness of, for example, 6 to 70 μm, preferably8 to 35 μm. The conductor layer is formed of conductor foil, preferablycopper foil.

Rather than provision of a conductor foil on the insulating film 11, aninsulator material, such as a polyimide precursor, is applied to aconductor foil, followed by heating so as to form an insulating filmmade of polyimide.

The conductor layer provided on the insulator film 11 is patternedthrough photolithography. Specifically, a photoresist is applied to theupper surface of the conductor layer and is subjected to light exposureand development. The conductor layer is etched (chemically dissolved)with an etchant through a patterned photoresist layer serving as aphotomask. The remaining photoresist is removed through dissolution withan alkaline liquid or similar material, thereby patterning the conductorlayer to form the wiring pattern 12 on the insulating film. During theabove steps, the dummy patterns 23 to 26 are also formed.

On the wiring pattern 12, a tin plate layer or a similar layer is formedin accordance with needs. In some cases, a metallic layer (e.g., anickel plate layer or a gold plate layer) is formed on the inner leads21 a and 22 a and the outer leads 21 b and 22 b, depending on theconditions under which connection with electronic devices or otherdevices is performed. However, provision of the aforementioned metalliclayer is not essential in the present invention, and should not beconstrued as limiting the invention thereto. Needless to say, the dummypatterns 23 to 26 may be formed exclusively of the conduct layer withoutplating, or may also be formed of a plated conduct layer.

As described hereinabove, in the flexible printed wiring board 10 ofEmbodiment 1, grid-like dummy patterns 23 to 26 extending in thelongitudinal direction of the tape are provided in blank areas A to D,which are present on both sides of the wiring pattern 12 in the widthdirection of the tape. Therefore, almost uniform rigidity of the tape inthe width direction is attained. In other words, uniformity inflexibility is attained, thereby reducing warpage. According to thepresent invention, the tape has more uniform flexibility, as comparedwith the case in which the blank areas A to D are completely coveredwith a conductor layer (i.e., solid-coated), with the conductor layernot having been patterned. Thus, even when the warpage is straightenedor reversed during conveyance of the tape in production and inspectionsteps, stress concentration is prevented. Accordingly, during mountingof electronic devices such as IC chips and LSI chips, wire breakage inthe inner leads 21 a and 22 a and cracking in solder resist can beprevented.

Embodiment 2

FIG. 2 schematically shows a plan view of a flexible printed wiringboard of Embodiment 2. The same members as shown in FIG. 1 are denotedwith the same reference numerals, and repeated descriptions are omitted.

The flexible printed wiring board 10A of Embodiment 2 has a wiringpattern 12A, which is slightly different in shape from the wiringpattern 12 of Embodiment 1.

Furthermore, Embodiment 2 differs from Embodiment 1 in that each of thestrut portions 18A in flexural slits 15A and 16A is provided such thatthe strut portion is shifted, in the width direction of the tape, fromthe position of each end of the device hole 14 in the width direction ofthe tape. Specifically, in Embodiment 1, each strut portion 18 in theflexural slits 15 and 16 is provided such that the position of eachdevice hole 14 end almost coincides, in the width direction of the tape,with that of the strut portion, whereas in Embodiment 2, strut portionsand device hole ends are arranged so as not to be aligned in thelongitudinal direction of the tape. Through employment of thisconfiguration, even when the warpage is straightened or reversed duringconveyance of the tape, stress concentration in the portions surroundingeach end of the device hole 14 in the width direction of the tape isprevented. In addition, even when the tape is bent at the flexural slits15A and 16A, stress concentration in the inner leads 21 a and 22 a iseffectively prevented.

Embodiment 3

FIG. 3 schematically shows a plan view of a flexible printed wiringboard of Embodiment 3. The same members as shown in FIG. 2 are denotedwith the same reference numerals, and repeated descriptions are omitted.

The flexible printed wiring board 10B of Embodiment 3 has dummy patterns23A to 26A, which are slightly different in pattern shape from the dummypatterns 23 to 26 of Embodiment 2.

In the dummy patterns 23A to 26A, dummy wirings thereof extending in agrid-like manner in an oblique direction with respect to thelongitudinal direction of the tape are provided in blank areas A to D,which are present on both sides of the wiring pattern 12A in the widthdirection of the tape. The dummy patterns 23A and 24A (also 25A and 26A)have oblique angles different from each other and have been formed fromdummy wirings having such oblique angles that the wirings are arrangedwith linear symmetry with respect to the longitudinal direction of thetape. Through employment of this configuration, almost uniform rigidityof the tape in the width direction is attained. In Embodiment 3, theoblique angles of dummy wirings are different between the dummy patterns23A and 25A (also 24A and 26A). However, dummy wirings may be arrangedin the same direction between two dummy patterns, so long as the dummypatterns are provided with line symmetry with respect to thelongitudinal direction of the tape. No particular limitation is imposedon the oblique angles of dummy wirings.

Similar to Embodiments 2 and 3, no particular limitation is imposed onthe areas where the dummy patterns 23A to 26A are to be provided and onthe width and pitch of the dummy wirings, and these conditions may beappropriately predetermined in consideration of the shape of the wiringpattern 12A and wiring density. Specifically, the width and pitch of thedummy wirings are not particularly limited, so long as the wiringpattern 12A and the dummy patterns 23A to 26A, as a whole, providerigidity, in the width direction, of the highest possible degree ofuniformity.

Other Embodiments

In the aforementioned embodiments, the wirings are juxtaposed in thewidth direction of the tape. However, the wirings may be juxtaposed inthe longitudinal direction of the tape, or juxtaposed in an obliquemanner with respect to the longitudinal direction of the tape. In thesecases, when grid-like dummy patterns are provided in a width directionwith respect to the longitudinal direction of the wirings, particularlyon both sides of the wiring pattern, the same effects of the presentinvention can be attained.

In the aforementioned embodiments, one unit of the flexible printedwiring board has one device hole. However, needless to say, cases inwhich one wiring board unit has two or more device holes also fallwithin the scope of the present invention.

EXAMPLES Example 1

A flexible printed wiring board (3-layer TAB tape) having a structuresimilar to that of the flexible printed wiring board of Embodiment 1 wasproduced. The thus-produced flexible printed wiring board had a tapewidth of 70 mm and a tape length per film carrier unit of 57 mm. Theoutput outer leads 21 b were formed from 400 lead wirings (pitch: 80μm), whereas the input outer leads 22 b were formed from 200 leadwirings (pitch: 250 μm). Also, the output inner leads 21 a were formedfrom 400 lead wirings (pitch: 60 μm), whereas the input inner leads 22 awere formed from 200 lead wirings (pitch: 80 μm). Each of the dummypatterns 23 to 26 was formed from 30 dummy wirings (pitch: 750 μm).

Example 2

The procedure of Example 1 was repeated, except that the structure ofEmbodiment 2 was employed, to thereby produce a flexible printed wiringboard.

Comparative Example 1

The procedure of Example 1 was repeated, except that the unpatternedconductor layer was left in the areas corresponding to the dummypatterns 23 to 26, to thereby produce a flexible printed wiring board.

Comparative Example 2

The procedure of Example 2 was repeated, except that the unpatternedconductor layer was left in the areas corresponding to the dummypatterns 23 to 26, to thereby produce a flexible printed wiring board.

Comparative Example 3

The procedure of Example 1 was repeated, except that the conductor layerwas completely removed in the areas corresponding to the dummy patterns23 to 26, to thereby produce a flexible printed wiring board.

Comparative Example 4

The procedure of Example 2 was repeated, except that the conductor layerwas completely removed in the areas corresponding to the dummy patterns23 to 26, to thereby produce a flexible printed wiring board.

Test Example

Each of the flexible printed wiring boards of the Examples and theComparative Examples was produced as a 125-m length tape, and IC chipswere mounted on the tape. After mounting, all products were inspectedfor wire breakage by checking electric conduction. Incidence of wirebreakage was calculated on the basis of the following formula:Incidence of wire breakage(%)=100×(count of defective products)/(totalnumber of inspected samples).

The results are shown in Table 1. TABLE 1 Incidence Positions of strutof wire portion and device breakage Space hole end (%) Ex. 1 dummypattern aligned flush 1.6 Ex. 2 dummy pattern offset 0 Comp.non-patterned aligned flush 5.3 Ex. 1 conductor layer Comp.non-patterned offset 3.4 Ex. 2 conductor layer Comp. none aligned flush4.8 Ex. 3 Comp. none offset 3.1 Ex. 4

As is clear from Table 1, the flexible printed wiring board of Example2, in which the dummy patterns 23 to 26 were provided and each of thestrut portions 18 in the flexural slits 15A and 16A was provided suchthat the strut portion is shifted, in the width direction of the tape,from the position of each end of the device hole 14, exhibited no wirebreakages. The flexible printed wiring board of Example 1, in which thedummy patterns 23 to 26 were provided but the position of each strutportion 18 is not shifted, exhibited an incidence of wire breakage asremarkably low as 1.6%.

In contrast, the flexible printed wiring boards of Comparative Examples1 and 2, in which no dummy pattern was provided and an unpatternedconductor layer was left, and the flexible printed wiring boards ofComparative Examples 3 and 4, in which the conductor layer wascompletely removed from the areas where the dummy patterns were to beformed, exhibited an incidence of wire breakage as high as 3% or higher,due to lack of uniformity in rigidity.

In the flexible printed wiring board of the present invention, grid-likedummy patterns are formed in a width direction generally symmetricallywith respect to the longitudinal direction of the flexible printedwiring board. Therefore, the flexible printed wiring board has uniformflexibility, and stress generated, for example, at straightening orreversing of warpage during conveyance of the wiring board duringproduction thereof is mitigated and released. Thus, wire breakage ininner lead portions and cracking in solder resist which would otherwisebe caused during mounting of devices such as IC chips and LSI chips canbe prevented.

1.-15. (canceled)
 16. A flexible printed wiring board comprising aninsulating layer, which insulating film has, along longitudinal edgesthereof, rows of sprocket holes at predetermined intervals; a wiringpattern formed of a plurality of juxtaposed wirings, which wiringpattern is formed through patterning a conductor layer stacked on atleast one surface of the insulating layer and on which wiring pattern asemiconductor chip is to be mounted; and grid-like dummy patterns formedin a blank area where the wiring pattern is not provided, which blankarea is present between the wiring pattern and the rows of sprocketholes, wherein the dummy patterns are formed in a width directiongenerally symmetrically with respect to the longitudinal direction ofthe flexible printed wiring board.
 17. A flexible printed wiring boardaccording to claim 16, wherein each of the grid-like dummy patterns areprovided in a direction of juxtaposition of the juxtaposed wirings. 18.A flexible printed wiring board according to claim 16, wherein thewiring pattern is formed such that the wirings are provided along thelongitudinal direction of the flexible printed wiring board, and thedummy patterns are provided on both sides of the wiring pattern withrespect to the direction of juxtaposition of the juxtaposed wirings. 19.A flexible printed wiring board according to claim 17, wherein thewiring pattern is formed such that the wirings are provided along thelongitudinal direction of the flexible printed wiring board, and thedummy patterns are provided on both sides of the wiring pattern withrespect to the direction of juxtaposition of the juxtaposed wirings. 20.A flexible printed wiring board according to claim 16, which is a widewiring board having a width of 48 mm or more.
 21. A flexible printedwiring board according to claim 17, which is a wide wiring board havinga width of 48 mm or more.
 22. A flexible printed wiring board accordingto claim 18, which is a wide wiring board having a width of 48 mm ormore.